"This ancient and mysterious charm conjures a magical guardian, a projection of all your most positive feelings. The Patronus Charm is difficult, and many witches and wizards are unable to produce a full, corporeal Patronus, a guardian which generally takes the shape of the animal with whom they share the deepest affinity. You may suspect, but you will never truly know what form your Patronus will take until you succeed in conjuring it."

The Patronus Charm (Expecto Patronum) is an immensely complicated, very difficult spell that evokes a partially-tangible positive energy force known as a Patronus (pl. Patronuses[4]) or spirit guardian[3]. The charm was primarily designed for defence against Dementors and Lethifolds, against which there is no other protection[5]. Albus Dumbledore, however, invented a means of using one's Patronus as a messenger to other witches and wizards, and this is the method of communication that the Order of the Phoenix uses[3].

It is evident from old woodcuts and scrolls that the Patronus Charm has been used since ancient times[6]. It therefore is unknown who created it, or when it was first invented[6].

To successfully cast the spell, one must muster the happiest memory they can think of (the happier the memory, the better the charm will work)[1] and begin drawing circles with their wand so as to increase the power of their spell[6]. They must then say the incantation, "Expecto Patronum"; the Patronus will come from the tip of the wand and can be directed towards a target by pointing one's wand at said target[7].

Difficulty

The Patronus Charm is widely regarded as advanced magic (so much so that its not even taught in Charms on the Hogwarts curriculum). It is a very complex charm and many qualified wizards and witches have trouble with it. In fact, Harry Potter is one of the youngest known wizards able to cast a Patronus; he was taught how to do so in early 1994 at the age of thirteen by Remus Lupin[8]. In a Dumbledore's Army lesson, Harry taught the members how to use the charm. Some were even successful in casting a corporeal form, though Harry said this might be because there was no Dementor to make them frightened[9]. However, some members were later shown to be able to cast corporeal Patronuses in the presence of Dementors[10].

Amelia Bones and potential members of the D.A. were all very impressed when Harry Potter revealed that he could cast not only a Patronus Charm but a corporeal one to boot, which is notably more difficult than casting an incorporeal one. Given that the success of the charm is directly reliant on the caster retaining a particular mental state the Charm is more difficult to cast in emotionally trying circumstances. For instance, Harry Potter, Ron Weasley, and Hermione Granger all had difficulty casting their Patronuses when under the negative influence of Salazar Slytherin's Locket, one of Lord Voldemort's Horcruxes, and after witnessing the death of Fred Weasley in battle[10]. This feature of the Charm is particularly unfortunate since the Charm's primary use is to defend against Dementors, Dark creatures specifically equipped to mentally unhinge people (and hence is partly why it is regarded as such advanced, difficult magic).

Self-confidence may also play an important role in casting the spell. In 1994 Harry is able to cast the spell successfully, driving off a large number of Dementors (which according to Severus Snape is only achievable by a very powerful wizard) thus saving Sirius and a younger version of himself, seemingly without recalling a happy memory, when he realises that he has already done it. Although Harry could have had a "happy memory" upon realising that he had saved all three of their lives.

Worthiness

Only wizards who are "pure of heart" are able to cast Patronuses. If the Patronus Charm is attempted to be cast by a Dark or otherwise not pure-hearted wizard, who would otherwise be magically capable of casting one, rather than the intended effect of a spirit guardian bursting forth, a horde of maggots will be conjured from the caster's wand, which will attempt to consume the caster. The first recorded instance of this happening is with the Dark Wizard Raczidian.[11]

For this reason, Lord Voldemort and his Death Eaters were not able to cast a Patronus Charm (not that it was necessary, as they already had the dark creatures under their control via their affinity to darkness, and would have no use for Patronuses).[12] However, Dolores Umbridge is able to produce a corporeal Patronus, despite the fact that she was considered an evil person; her wearing of Salazar Slytherin's Locket, which was one of Voldemort's Horcruxes, was what enhanced her strength due to her evil affinity. She was able to produce a corporeal Patronus due to the enhanced strength. Conversely, Draco Malfoy was unable to cast one despite his waning evil. Severus Snape is the only Death Eater capable of casting the charm, as his love for Lily Evans was his one redeeming point.

The Patronus

A successfully conjured Patronus can take two forms: non-corporeal or corporeal, and both types vary greatly in their appearances and strength.

Non-corporeal

Harry, casting a non-corporeal, shield Patronus.

A non-corporeal Patronus is one which does not resemble any living creature and has few distinguishing features (if any). Non-corporeal Patronuses resemble a burst of vapour or smoke without any clearly defined form shooting from the tip of the wand. While they may be partially effective at halting dementors (but not repelling them), non-corporeal Patronuses are not "fully-fledged" Patronuses, and are regarded as a more primitive or weaker version of the true Charm.

While even the most advanced wizards, like the prodigious Severus Snape, appear to have to use the spoken incantation Expecto Patronum to conjure a corporeal Patronus, Remus Lupin casts a non-corporeal Patronus on the Hogwarts Express as a Nonverbal spell. This suggests that only non-corporeal Patronuses can be conjured without the spoken incantation.

Corporeal

A corporeal Patronus is one that is fully-formed, taking the shape of bright-white, translucent animal. The specific animal forms that corporeal Patronuses take vary from person to person and reflect each individual's personality.

Harry casting a Patronus Charm when he was attacked in Little Whinging.

Patronus forms, however, are subject to change if the caster goes through an emotional upheaval of some sort, including falling in love. For example, Nymphadora Tonks's Patronus changed from its previous form to that of a four-legged creature which others guessed was a werewolf because then it would reflect her love of Remus Lupin, himself a werewolf[9].Another example is Severus Snape, whose Patronus was that of a doe, the same as Lily Evans, though this may have been the case all along rather than a change from its original form, as Snape was in love with Lily for most of his life. Based on the fact that Patronuses are conjured by recalling happy memories, it stands to reason that a Patronus might change its form after one falls in love and the memories used to conjure the Patronus largely revolve around the person the caster is in love with. In addition, it seems that some couples have "complementary" Patronuses (male and female forms of the same animal), such as Lily and James Potter. It is, however, unknown if this is a coincidence or not. If a wizard is an Animagus and can summon a corporeal Patronus, the two may take the same form, as was the case for Minerva McGonagall and James Potter[10].

The corporeal Patronus is also better at repelling Dementors than the non-corporeal form.

Uses

There are two known uses for the Patronus Charm. The first and main use of the Patronus is to drive off certain Dark creatures such as Dementors and Lethifolds, and is the only known charm that will work against them. As Dementors feed on happy memories to render humans to drown in sorrow and sadness, the Patronus acts as a shield of sorts, of which the Dementor would try to feed off of instead - the Patronus is comprised exclusively of positive feelings and cannot comprehend negative emotion, so the Dementor's influence cannot harm it.

Harry casting a Patronus during the Hunt for Horcruxes

The other use was devised by Albus Dumbledore, who invented a way of using the Patronus (corporeal and non) as a means of communication. They can deliver messages, speaking with the voice of the caster. This source of communication is believed to be exclusive to the members of the Order of the Phoenix. Using the Patronus for communication offers great advantages in security, as Patronuses identify the caster and are Dark-proof.[9] A good example of this was when Minerva McGonagall used her feline Patronus to call for help to the other heads of houses in 1998 when she found out Harry Potter had returned to Hogwarts in search of Rowena Ravenclaw's Diadem. Multiple Patronuses can be cast for communication, as McGonagall was able to create three. Another is when Severus Snape used his Patronus to lead Harry Potter to a pool in the Forest of Dean in 1997, where he had laid Godric Gryffindor's Sword for Harry to retrieve.

Etymology

Patronus means "protector", "guardian", or simply "patron" in Latin, specifically reflecting the entire role that the Patronus Charm plays. In archaic Latin, it meant "father", which is very interesting, considering that Harry Potter's Patronus is the same as his father's Patronus and Animagus form, a stag. The Latin word exspecto or expecto means "I look out for" or "I wait for", thus the charm's incantation roughly translates into "I await a protector."

Known Practitioners

"A Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him. In order for it to work, you need to think of a memory. Not just any memory, a very happy memory, a very powerful memory… Allow it to fill you up... lose yourself in it... then speak the incantation "Expecto Patronum"."

Initially, Illyius's fellow villagers mocked him as they had never seen such a small and weak-looking Patronus, which corresponded to Illyius's timidity and tongue-tiedness. However, when dark wizard Raczidian invaded the village with his army of Dementors, Illyius's mouse Patronus proved to be stronger than the others, making him a hero.

The bird whose loyalty he earned, Fawkes, was a phoenix. Also he used it in 1995 as a message to Hagrid when Krum was stunned. It was described as "a ghostly bird that flew off in the direction of Hagrid's house".

The form of Tonks' Patronus changed in 1996. It became a werewolf due to her love for Remus Lupin, though Harry Potter and Hermione Granger initially believed it was a black dog, triggered by the death of her second cousin Sirius Black (whose Animagus form was a black dog). Changes of Patronus are rare and usually caused by devastating life traumas. Occasionally when a witch and a wizard are married, their Patronuses will match each other (most likely because the witch/wizard's happy thought will be that of their spouse).

While interrogating Muggle-borns at a trial, she had cast a Patronus to ward the Dementors from the prosecutors. Her favourite animal, as shown by the many plates with cats on them in her office. She was able to cast the Patronus due to her affinity with the locketHorcrux's evil influence.

Behind the scenes

Two Patronuses planned for the films but ultimately cut include a Thestral and a chimpanzee[16]

The Patronuses of James Potter and Minerva McGonagall match their respective Animagi form. These are the only characters whose Patronus and Animagus forms are known. It is uncertain whether all Patronuses will match the form of the Animagus, but so far, an instance of differing forms has not been revealed.

In the video game adaptation of Harry Potter and the Prisoner of Azkaban, the more moderate Expecto Patronum is a ball of light (before cast, a white ring goes up Harry's arm and if it reaches top of the wand, Harry has to try the spell again) and is used only to attack Dementors. At the climax of the game, a stag (Harry's Patronus) jumps out of the wand to drive away all the Dementors nearby attacking Harry and Sirius.

In Harry Potter and the Deathly Hallows: Part 1, Dolores Umbridge's Patronus seems to emanate a shield that keeps the Dementors separated from everyone in the courtroom. In addition, Kingsley Shacklebolt's Patronus is described in the novel as taking the form of a lynx, however in the film it is depicted as a shimmering comet of light from which springs silvery images of witches and wizards screaming in terror as Kingsley's voice details the death of Scrimgeour and the fall of the Ministry.

According to W.O.M.B.A.T., it is possible that Patronuses vary in strength according to which animal's form they take. Later canon from Wonderbook: Book of Spells, however, stresses that this is not the case and that the form of the animal has no correlation to the strength of the Patronus (see, for instance, Illyius and Symposia Rawle).

In an interview, J. K. Rowling stated that the happy memory that would produce the strongest Patronus for her would be the births of any of her three children, though a close fourth would be when she learned that Harry Potter and the Philosopher's Stone was going to be published.[18] She has also stated that she'd like for her Patronus to be an otter, like Hermione's, but she has a feeling it might actually be a large dog.[17] Additionally, in a 2000 interview, she stated that the Patronus Charm was her favourite spell in the series so far.[19]

Although Lupin provides most of the information within the books about the Patronus Charm, his Patronus is never identified.

Luna Lovegood's hare Patronus is possibly a reference to the moon rabbit, a figure in East Asian mythology.

Learning to cast a Patronus is one of the tasks given to the player in the final Chapter of the game Wonderbook: Book of Spells. For learning the incantation and gesture, the player is given the Trophy "Expecto Patronum!" On the next page, the player is given the chance to cast a true Patronus, the form of which is apparently chosen randomly. Amongst the possible Patronuses are a bat, a bull, a cat, a goat, a horse, a rabbit and a scorpion.[20] For discovering their Patronus, the player is awarded the Trophy "True Patronus."

There was a noted change in appearance of Patronuses in general in the Order of the Phoenix movie. In movies 5-8, Patronuses very wispy and airy, while in the third film they look more substantial, solid and brighter.

In Harry Potter and the Deathly Hallows: Part 1, during the infiltration of the Ministry of Magic, Umbridge's Patronus was very clearly seen several times in the same scene and often looked less like a cat and more like a reanimated, feline corpse, with the skeletal features visible. This is odd for a Patronus (they usually look quite 'alive' and well) and its twisted form may be a nod to both Umbridge's nature and the circumstances under which it was cast.